Spin Asymmetries and Helicity Amplitudes in Photo-Production from Polarized Neutrons

Abstract

Pseudoscalar meson photoproduction experiments lead the search for an expanded $$N^*$$ spectroscopy that is required by lattice solutions of QCD. Extensive data have been collected with proton targets that can potentially over-determine the production amplitude and allow a search for weak resonances. In contrast, data on photoproduction from neutrons is drastically limited. Both are needed since the $$\gamma p N^*$$ and $$\gamma n N^*$$ photo-couplings to $$I=1/2$$ states are different and provide complementary information on the mechanisms for resonance excitation. The main subtleties in experiments with an effective polarized neutron target are reviewed. The considerable impact of new polarization data on multipole analyses are discussed, using the example of recent $$\pi ^- p$$ measurements of the beam-target helicity asymmetry (E) from polarized $${ HD}$$ in CLAS at Jefferson Lab. New partial wave analyses incorporating these data have found significant changes in helicity amplitudes of some established resonances, and signatures of weak resonances that previously were observed only in hyperon decay channels.